This invention relates to high intensity discharge lamps and in particular to such lamps which include an amalgam for self-regulation of input power in spite of line voltage changes.
At present, power regulation in high intensity arc discharge lamps requires relatively complicated and expensive electronics. There is no mechanism internal to the lamp for compensating for such line fluctuations which do occur regularly on conventional alternating current power lines.
In conventional high intensity discharge lamps in which the gaseous discharge medium comprises a mixture of a rare gas and mercury or rare gas along with mercury and a metal halide vapor, the mercury vapor pressure is limited. That is, all of the mercury in the arc tube is vaporized. The voltage drop along the arc in these conventional high intensity discharge lamps is determined by the mercury vapor pressure and is constant and is essentially proportional to the amount of mercury between the electrodes, that is, the mercury vapor density. Since this is fixed and limited by the amount of mercury originally incorporated into the lamp during manufacture, the arc voltage drop is constant for a given applied voltage. However, along with line voltage fluctuations, the power supplied to the lamp and therefore the intensity of the light output from the lamp varies. This variance of light output from the lamp is highly undesirable and as mentioned above is controllable only through expensive electronics.
In some fluorescent lamps, amalgams are used to control the mercury vapor pressure to achieve optimum production of 254 nanometer ultraviolet radiation. This control, however, is not accompanied by any change in the electrical characteristics of the arc discharge; the change in pressure is related only to the optical characteristics of the plasma to insure maximum production of the resonance radiation. Amalgams, however, have not been employed in high intensity discharge lamps.